1. Field of the Invention
The present invention relates to an exposure mask, a method of manufacturing the same, and an exposure method using the same and, more particularly, to an exposure mask used for lithography.
2. Description of the Related Art
In the past few years, the packing density and the degree of micropatterning of semiconductor ICs have steadily increased. In the manufacture of semiconductor ICs, lithography is especially important process, and consequently, various lithography techniques, using G-line rays, i-line rays, an excimer laser, x-rays, and the like as light sources have been evaluated. Similar study has been carried out with regard to resists, so as to develop new resists and new resist treatments, such as the REL method. It has been carried out with regard to Si wafers as well, so as to develop new wafer treatments, such as the three-layer resist method, the CEL method, the image reverse method, and the like.
In contrast to this, techniques of manufacturing masks have received less study. However, recently, a phase shift method for improving the resolution of mask patterns has been proposed and has received a great deal of attention (IBM 1986, IEEE Trans. Elect. Devices. Vol. ED-29, 1982, CP 1828; 1988, Fall Applied Physics Meeting 4a-K-7, 8 (p497)). FIGS. 1A to 1C show a mask manufacturing process for photolithography using this phase shift method.
In this process, as shown in FIG. 1A, a light-shielding layer 11 consisting of Cr (chromium) or Cr.sub.2 O.sub.3 (chromium oxide) is deposited on a quartz mask substrate 10 to a thickness of about 100 nm by sputtering. A resist 12 is then patterned.
As shown in FIG. 1B, the light-shielding layer 11 is etched by wet etching or reactive ion etching, using the patterned resist 12 as a mask.
Subsequently, as shown in FIG. 1C, shifters 13 for shifting the phase of incident light are formed by using, for example, a resist. In this case, alternate openings in a line/space portion are covered with the phase shifters 13. In an isolated space portion, auxiliary patterns 14, which cannot be individually resolved, are formed on both the sides of an opening, and are covered with the phase shifters 13.
Since, when exposure is performed by using such a mask, the phases of light beams passing through the respective openings are alternately inverted, as indicated by dotted lines in FIG. 2B, the photo-intensity on a portion immediately under each light-shielding layer 11 is greatly decreased. As a result, the photo-intensity distribution indicated by a solid curve in FIG. 2B is realized. With this mask, IC patterns can be formed having a dimensional resolution of about 1/2 that of IC patterns formed with a conventional mask.
However, the following problems have arisen in connection with the process of manufacturing the phase shift type photolithography mask described above.
Firstly, after the mask pattern is formed, the shifters 13 are arranged on the alternate openings formed in the line/space portion, while the auxiliary patterns 14 are formed on the light-shielding layers 11 in the isolated space portion, the shifters 13 then having to be arranged on the patterns 14. Therefore, at least two pattern formation steps and an alignment step are required for forming the mask pattern and the phase shifters. However, since a mask drawing electron-beam exposure unit generally does not process an alignment function, an exposure unit capable of alignment, such as a direct drawing EB exposure unit, must be specially developed. This entails considerable labor, time, and cost.
Secondly, a large amount of complicated data processing is required for the EB data of the mask pattern and the shifter lithography data.
Thirdly, the auxiliary patterns 14 must be formed having a pattern size smaller than the minimum pattern size of a mask.
Fourthly, light passing through the auxiliary patterns 14 may not be sufficiently cancelled, and hence a pattern may be distorted.
Lastly, patterns each having the same resolution cannot be formed in an isolated line portion, an island portion, and the like, such as other patterns can be.
As described above various problems arise in connection with the process of manufacturing conventional phase shift type masks for photolithography; for example, a new EB mask exposure unit must be specially developed, a large amount of complicated data processing is required in relation to the EB data of a pattern and the shifter process data, auxiliary patterns must be formed having a pattern size smaller than the minimum pattern size of a mask, a pattern formed in an isolated space portion may be diltorted, and it is difficult to set the same resolution for all the patterns to be formed.